While the invention is subject to a wide range of applications, it is especially suitable for use in a system for transmitting data concurrently with the transmission of music and voice programs using the same transmitting and antenna structure as a conventional amplitude modulation (AM) broadcast station.
There have been a number of methods proposed for transmitting data along with an AM broadcast signal. Most of these methods transmit data at relatively slow speeds. Generally, the data is transmitted by phase or frequency modulating the carrier and then this angular modulated wave is amplitude modulated by the normal music and voice program material. The resulting composite modulated wave can then be demodulated with an envelope demodulator to extract the normal program material. Since the envelope demodulator is insensitive to the phase of the composite wave, listeners are unaware of the data modulation. Indeed, secret transmissions have been reported to have been made with such a system during World War II.
However, the rate of information flow through such systems have generally been very slow. If higher data rates are attempted, the bandwidth of the composite wave will be noticeably wider than normal AM broadcast signals because each sideband generated by the phase or frequency modulation is then surrounded by sidebands produced by the amplitude modulation process.
There are two basic types of interference that are pertinent to the instant invention.
The first is self interference, specifically interference to those wishing to receive the normal broadcast program on the one hand and interference to data reception on the other.
The second type of interference is interference to listeners to other stations, both adjacent or co-channel stations.
Considering first the self interference and, more specifically, interference to the normal broadcast program listeners, it is important that the data signal not be detectable.
The instant invention accomplishes substantially interference-free operation by a number of mechanisms. First of all, and in common with the prior art, the modulation for the data is substantially a form of angular modulation; i.e., quadrature modulation. While quadrature modulation includes an inphase (envelope) component which can be detectable by envelope detectors, the amplitude is small. For example, if each of the quadrature modulation sidebands is restricted, to a say 10% of the carrier amplitude, the resulting envelope modulation is approximately 1%. It must be stressed, however, that errors in receiver tuning, multipath conditions, etc. can convert the quadrature sidebands to larger in-phase components. Fortunately, under most conditions such problems will not cause any difficulty.
In one embodiment of this invention, as shown in FIG. 1 and described below, it is seen that means are provided for controlling the amplitude of the quadrature modulation sidebands as a function of the program amplitude modulation. Thus, when the normal program is absent, the data quadrature modulation sidebands are reduced to zero amplitude. However, as the amplitude modulation increases, the radiated level of the data sidebands is increased so that, for one embodiment of the invention, the quadrature modulation sidebands are always at least approximately 15 db below the level of the program amplitude modulation sidebands. This provides a masking effect for listeners to the normal broadcast program in addition to the isolation provided by quadrature modulation and, for all practical purposes, the data sidebands do not interfere, under normal conditions, with the broadcast channel.
This invention may be used to transmit both monophonic and stereophonic broadcast program material. All proposed methods of transmitting stereo require both in-phase and quadrature modulation components. In the stereo systems, the L-R components produce angular modulation. Thus, the demodulation means for such stereo signals is responsive to angular modulation and would be subject to interference by the data quadrature modulation components. In at least one presently operating AM Stereo system, the ISB system, as described in U.S. Pat. Nos. 3,908,090 and 4,373,115 uses a mixed highs (i.e., where stereo separation is substantially reduced or eliminated above a frequency, say, in the order of 6 to 8 kHz) method of operation is provided. At some frequency, generally 6 to 7 kHz, the stereophonic separation is reduced substantially. Accordingly, the sensitivity of the receiver to frequencies above 6 or 7 kHz to angular modulation can be greatly decreased without altering the stereo performance.
In order to maintain the low interference characteristic for stereo reception of the amplitude modulated signal, the data is transmitted preferably in the frequency range where the "mixed highs" technique is functioning. Accordingly, the data is quadrature sidebands at a frequency of 10 kHz in the United States and 9 kHz in certain other countries.
By the use of the mixed highs approach the amount of interference suffered by data signal receivers is also minimized because the broadcast material has little or no angular modulation at the frequencies to which the data receiver must respond. The data receiver transmission system would best use modulation techniques that can produce low data error counts even when subject to relatively poor signal-to-noise and interference situations. It is also, of course, possible to use various error correcting codes or at least error sensing codes plus redundancy to further decrease data error counts.
The second type of interference; i.e., interference to adjacent channels may be maintained within acceptable levels by always maintaining the data sidebands well below the level of the AM broadcast signal.
A general object of the present invention is to provide a system for transmitting data concurrently with normal broadcast programs over a standard AM broadcast station.
A further object is to achieve such concurrent data modulation without disturbing listeners to the normal broadcast programs.
A still further object is to provide data transmission without causing significant additional interference to other broadcast stations.
An additional object is to permit higher speed data transmission with low error rates.
Another ojbect is to porvide suitable data receivers for use with such a system.
An additional object is to transmit the data at a specific frequency that minimizes interference to adjacent channel stations.